A printing machine is used in the packaging industry to print plate elements, such as sheets of paper or cardboard. The machine includes a plurality of successive stations. A first station located most upstream is an infeed station successively introducing the sheets one after the other from the bottom of a stack. The infeed station feeds a plurality of printing stations in the form of one or more printing units placed one after the other. Each of the printing units prints one color. A delivery station that collects the printed sheets is provided at the end of the machine.
In the case of printing sheets of cardboard, more particularly corrugated cardboard, the technology most frequently employed is flexographic printing. A flexo machine includes one or more printing units as a function of the number of colors required. A printing unit notably includes a plate cylinder around which is wrapped and on which is tensioned a flexible plate. This plate prints the sheet after it has been coated with ink using a screened cylinder known as the anilox cylinder and an inking device. The sheet passes between the plate cylinder and an pressure roller. The printing is effected on the bottom of these sheets, so the sheets are transported from the top. A plate cylinder prints one or more patterns using the same color during each of its rotations.
The plates are produced with a screen that consists of a rectangular array of larger or smaller dots. This screen is used to transform a half-tone original, for example a photo, into a printable dotted image. Screens with round dots are most frequently used in flexographic printing. The dots enable better reproduction of details and a better transition between tones.
To obtain a final image of good quality on the printed sheet, it is notably necessary for all the patterns in different colors to be superposed exactly. It is also necessary that the screen dots are not deformed, for example from a round shape to an oval shape.
The sheets are transported by a vacuum system using a belt or flat belts or steel rollers driven to move the sheets longitudinally from one printing unit to another in the upstream to downstream direction, from the infeed station to the delivery station. To obtain printing of optimum quality, the basic principle is that the sheets are transported at the most regular speed possible.
However, the belt(s) wear and stretch and the sheet transport speed is no longer regular from one job to another. The performance of the machine is degraded. Maintenance of the transport system takes time and involves stopping production completely. In the case of roller conveyors, it is the non-homogeneous nature of the sheets to be printed or being printed that causes speed variations. Ovalization or deformation of the printed screen dots can be seen on the printed sheets.